Microbial inoculants as a soil remediation tool for extensive green roofs

•Addition of soil microbes shown to affect higher trophic levels for the first time•Trichoderma shown to alter green roof soil microarthropod communities•Microbial inoculants found to have no benefit to mature Sedum spp . on green roofs

Green roofs are increasingly used in the urban environment to insulate buildings, reduce stormwater runoff and remediate biodiversity lost in construction. Most common in the Northern Hemisphere are extensive green roofs, due to their low-cost and low-maintenance requirements. However, plant growth on these roofs is often limited and this could have implications for ecosystem service provision as well as reduce the economic feasibility of green roofs as an aesthetically successful product. In addition, the increasing popularity of green roofs as an eco-product means that a high number of these roofs, that do not reach their maximum potential in terms of plant growth, already exist, highlighting a need for a successful remediation tool post-build.Previous studies suggest that the soil food web on green roofs, integral for nutrient cycling in soils, is also lacking and that this may be an effective aspect to target in order to improve plant establishment and success. Microbial inoculants have already been added to green roofs, but with little scientific research informing their application. In this field experiment we aimed to determine if the addition of these foundation species in green roof soil food webs, including mycorrhizas, Trichoderma spp . and soil bacteria, could improve the abundance and biodiversity of higher trophic species, such as microarthropods, and if this had resultant effects on plant growth on a mature green roof.It was found that some microbial inoculants were more successful at remediating soil food webs than others, with Trichoderma in particular producing higher populations of some microarthropod groups. However, these changes in microarthropod community dynamics did not have a resultant positive effect on Sedum spp . growth. The authors hypothesize that mature green roofs have an established microbial community that may limit the success of commercial inoculants. This is the first study to demonstrate multi-trophic community changes as a result of the addition of soil microbial inoculants.